rt8011 Richtek Technology Corporation, rt8011 Datasheet - Page 13

• Manufacturer Part Number: rt8011
• Description: 4mhz, Synchronous Step-down Regulator
• Manufacturer: Richtek Technology Corporation

Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as :
Efficiency = 100% − (L1+ L2+ L3+ ...) where L1, L2, etc.
are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses: V
The V
loss at very low load currents whereas the I
dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve
at very low load currents can be misleading since the
actual power lost is of no consequence.
1. The V
the DC bias current as given in the electrical characteristics
and the internal main switch and synchronous switch gate
charge currents. The gate charge current results from
switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
high to low to high again, a packet of charge ΔQ moves
from V
of V
continuous mode, I
are the gate charges of the internal top and bottom
switches.
Both the DC bias and gate charge losses are proportional
to V
higher supply voltages.
2. I
internal switches, RSW and external inductor RL. In
continuous mode the average output current flowing
through inductor L is
and the synchronous switch. Thus, the series resistance
looking into the LX pin is a function of both top and bottom
MOSFET R
RSW = R
for both the top and bottom MOSFETs can be obtained
from the Typical Performance Characteristics curves. Thus,
to obtain I
DS8011/A-00 August 2006
2
DD
DD
R losses are calculated from the resistances of the
DD
DD
that is typically larger than the DC bias current. In
and thus their effects will be more pronounced at
DD
quiescent current loss dominates the efficiency
DS(ON)
DD
to ground. The resulting ΔQ/Δt is the current out
2
R losses, simply add RSW to RL and multiply
quiescent current is due to two components :
DS(ON)
quiescent current and I
TOP x D + R
and the duty cycle (D) as follows :
GATECHG
“chopped”
= f(QT+QB) where QT and QB
DS(ON)
between the main switch
BOT x (1"D) The R
2
R losses.
2
R loss
DS(ON)
the result by the square of the average output current.
Other losses including C
losses and inductor core losses generally account for less
than 2% of the total loss.
Thermal Considerations
In most applications, the RT8011/A does not dissipate
much heat due to its high efficiency. But, in applications
where the RT8011/A is running at high ambient
temperature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance. To avoid the RT8011/A from exceeding
the maximum junction temperature, the user will need to
do some thermal analysis. The goal of the thermal analysis
is to determine whether the power dissipated exceeds
the maximum junction temperature of the part. The
temperature rise is given by : T
the power dissipated by the regulator and θ
resistance from the junction of the die to the ambient
temperature. The junction temperature, T
T
As an example, consider the RT8011/A in dropout at an
input voltage of 3.3V, a load current of 2A and an ambient
temperature of 70°C. From the typical performance graph
of switch resistance, the R
at 70°C is approximately 121mΩ. Therefore, power
dissipated by the part is :
P
For the DFN3x3 package, the θ
junction temperature of the regulator is : TJ = 70°C +
(0.484W) (110°C/W) = 123.24°C Which is below the
maximum junction temperature of 125°C. Note that at
higher supply voltages, the junction temperature is lower
due to reduced switch resistance (R
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to ΔI
J
D
= T
= (I
A
LOAD
+ T
R
)
LOAD(ESR)
2
Where T
(R
DS(ON)
, where ESR is the effective series
OUT
A
) = (2A)
is the ambient temperature.
immediately shifts by an amount
IN
DS(ON)
and C
2
(121mΩ) = 0.484W
R
JA
of the P-Channel switch
= P
is 110°C/W. Thus the
RT8011/A
OUT
D
DS(ON)
x θ
ESR dissipative
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JA
JA
).
J
, is given by :
is the thermal
Where PD is
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